Genesis, neurotrophin responsiveness, and apoptosis of a pronounced directconnection between the two eyes of the chick embryo: A natural error or a meaningful developmental event?

Unilateral intraocular injections of either of two fluorescent carbocyanine dyes into the embryonic chick eye resulted in both retrograde staining of ganglion cells (GGs) in the eye contralateral to site of injection and ante rograde labeling of axons whose cell bodies were located within the injecte d eye. This prominent retino-retinal projection formed by thousands of GCs having a nasal origin and temporal termination appeared at embryonic day 6 (E6), attained its maximum intensity at E13-E14, and gradually disappeared until E18. The axonal growth cones ended superficially and never penetrated deeper layers of the retina. Treatment of the projection with BDNF resulte d in massive terminal branching of the axons within deeper layers of the ta rget retina. Double injection into the eye and the isthmo-optic nucleus sho wed a concomitant ingrowth of axons in the contralateral retina. Individual GCs died between E9 and E13, but massive apoptotic cell death was mainly m onitored at E14 and later. Disintegrated cells showed typical images of apo ptosis. Because degenerating cells were prelabeled with the membranophilic fluorescent carbocyanine dye, their death allowed the concomitant visualiza tion of phagocytosing cells, too. Radial Muller glia were the only class of cells observed to become phagocytotic between E9 and E16. These cells beca me replaced exclusively with microglial cells from E17 on. The results sugg est that the topologically restricted retino-retinal projection may have so me developmental significance rather than representing a massive erroneous projection. Most likely, the projection may serve as a "template" to guide centrifugal isthmo-optic axons into the retina.